The present invention relates generally to apparatus for the detection and measurement of magnetic anomalies in magnetic materials and is more particularly directed to manually operated apparatus for detecting magnetic flux leakage such as used for inspecting for defects in the bottoms of chemical and petroleum storage tanks.
Chemical and petroleum storage tanks face gradual and continual deterioration due to the harsh chemical environment both outside and within the vessel. The steel walls of a tank are subject to corrosion, pitting, and other chemical and physical processes that can cause localized damage to the walls. Such localized damaged regions can develop into leaks or in extreme cases can lead to rupture of the tank. The tank bottom is exposed to corrosion or similar damage from the underside as well as from the top side. A tank typically rests on sand, gravel, crushed limestone, clay or similar base of varied composition. When the tank is filled, the bottom flexes and presses into the material under the weight of the contents. When the tank is then emptied, the bottom rises causing air and moisture to be drawn in, which accelerates the underside deterioration process.
To guard against environmentally damaging leaks or other tank failure, tank bottoms should be inspected periodically for early signs of damage conditions that may result in leakage. The underside of the tank bottom of course is inaccessible and thus cannot be inspected directly. One popular form of inspection apparatus looks for magnetic anomalies caused by local damage to the steel tank bottom. This apparatus includes one or more strong permanent magnets or electromagnets that induce a magnetic field within the steel plate forming the tank bottom that in effect locally magnetizes the plate. A region of the plate without defects produces an induced magnetic field of known form that is generally confined within the plate. Localized defects from corrosion, pitting and the like cause this generally confined magnetic flux to "leak out" of the steel plate where it may be detected by the inspection apparatus positioned just above the plate surface, even if the defect producing the magnetic anomaly is on the inaccessible underside of the plate. Thus, detecting a magnetic anomaly may signal the site of a defect in the steel bottom.
For good inspection sensitivity and accuracy with respect to corrosive type defects, it is desirable to apply the largest possible magnetizing force to the material. There is a practical limitation, however, to the magnitude of the largest magnetizing force that can be applied: The attractive magnetic force between the magnets in the apparatus and the material being magnetized can become unmanageably large. If the apparatus is to be usable, it must strike a balance between the magnitude of the magnetization induced in the material under inspection--a larger induced magnetization provides greater sensitivity to magnetic anomalies--and the strength of the magnetic attraction between this material and the apparatus--a larger magnetic attraction hinders maneuverability of the apparatus.
Typical magnetic inspection apparatus includes a carriage mounted on wheels that carries the magnet for inducing the field within the tank bottom, the sensors for detecting the flux leakage, a motor for driving the wheels, and various other subassemblies needed for the apparatus to function. To perform an inspection, the apparatus is wheeled slowly across the tank bottom while the on-board sensors search a strip, typically about twelve inches wide, for magnetic leakage flux. The apparatus is manually maneuvered over the tank bottom in this way until the entire bottom is covered.
The magnetic inspection apparatus is generally difficult to manipulate. The apparatus is heavy, typically weighing 100 to 300 pounds (44 to 130 kilograms). The necessarily strong magnetic attraction between the on-board magnet and the steel plate of the tank bottom adds substantially to the difficulty of freely moving the apparatus over the surface. Even with the on-board drive motor for the wheels, manipulating the apparatus in the course of inspecting a full tank bottom can be a laborious operation. A storage tank having an 80 foot (25 m) diameter, for example, may take up to eight hours to inspect.
Maneuvering the apparatus is laborious in part because the operator must first "break" the attractive magnetic force whenever it is desired to re-position the apparatus for inspecting a new region of the plate, for example when a sidewall is reached, or to navigate the apparatus around or over obstacles such as plate welds. Steel plate on the order of 1/4 to 1/2-inch thick (6 to 12 millimeters) is commonly welded to the tank bottom to patch previously discovered damage. When the edge of such patchwork is encountered, the operator must manually urge the apparatus over or around the welded edge to continue the inspection. Operators commonly find it burdensome to manipulate the apparatus back and forth over the tank bottom when the total attractive force exceeds about 200 pounds (about 90 kilograms) and extremely difficult if not prohibitively exhausting when the attractive force exceeds about 700 pounds (about 300 kilograms). At least one supplier has attempted to alleviate the burden on the operator by providing a foot pedal linked to the magnet assembly so that the operator may first displace the magnet away from the surface by depressing the foot pedal to break the magnetic attraction. In practice, however, the foot pedal still leads to operator fatigue over the course of several hours of inspection. Consequently, operator fatigue still places a practical limitation on the maximum magnetization that may be utilized, which in turn limits the sensitivity, accuracy and overall utility of the inspection apparatus.